A refractometer is a scientific instrument that is capable of determining the index of refraction of a liquid. Light bends when it travels from one medium into another medium due to a change of velocity of light as it passes through the interface between the media. When a light ray travelling in air enters a liquid medium, the light ray bends or changes direction by an amount dependent on the density of the liquid medium. The angle of refraction refers to the extent of this light bending phenomenon as the light exits one medium and enters another. Different liquids exhibit varying amounts of refraction according to their respective densities. For example, apple juice exhibits a different amount of refraction than orange juice because orange juice exhibits a substantially higher density than apple juice.
The index of refraction (nD or nS) of a particular liquid relates to the amount of light bending that the particular liquid exhibits. Larger amounts of bending by a medium correspond to higher indexes of refraction. A liquid that exhibits a low amount of bending exhibits a lower index of refraction than a liquid that exhibits a high amount of bending. Tables are available that correlate indexes of refraction to different materials. Tables are also available that correlate indexes of refraction to different concentrations of particular liquid media at a particular temperature. For example, in the case of a corn syrup liquid medium, different indexes of refraction observed for different corn syrup samples correspond to different concentrations of corn syrup. Thus, by using a refractometer to observe the index of refraction of a particular corn syrup sample, one can determine the concentration of the particular corn syrup sample by referring to a table or scale that correlates the index of refraction to concentration at a particular fixed temperature.
It is known that index of refraction readings vary with the temperature of the liquid medium. Thus, for index of refraction readings and corresponding concentration values to be accurate, the temperature of the sample should be the same temperature for which the scale is correlated.
Refractive index measurements have been used for process control in the food industry since the 1940s. Typical measurements are usually for sugars in fruits such as melons, for orange and other juices, for sugar content in grapes for the wine industry, and many other examples. Refractometers are also useful for determining the total dissolved solids (TDS) of liquids such as coffee and espresso.